Calculate burial depth and post length
In cold climates, posts must be buried below the frost line to prevent "frost heave" from pushing the posts out of the ground.
Adding 6 inches of gravel at the bottom of the hole allows water to drain away from the post, preventing rot.
Flaring the bottom of the hole (making it wider at the bottom) creates a "bell" shape that anchors the post more securely.
Scenario: You're building a 6-foot tall privacy fence in average clay soil. How deep should you dig the post holes?
Step 1: Calculate minimum depth = 6 ft × 0.33 = 2.0 feet
Step 2: Add gravel base = 2.0 + 0.5 ft = 2.5 feet recommended depth
Step 3: Calculate total post length = 6 ft + 2.5 ft = 8.5 feet total
Result: Dig holes 2.5 feet deep (30 inches). Buy 10-foot posts (allowing 18 inches extra for cutting to final height). You'll need approximately 1-2 bags of 80-lb concrete per post.
The 1/3 rule states that 1/3 of the total post length should be buried underground. For a 6-foot fence, you need 2 feet underground and an 8-foot post total. This provides adequate stability.
Frost line depth varies by region. Northern states like Minnesota have frost lines 4-5 feet deep, while southern states like Texas have minimal frost concerns (12-18 inches). Check local building codes for exact requirements.
Concrete provides maximum stability for heavy gates and tall fences. Fast-draining gravel (no concrete) works for lighter fences in well-draining soil and allows easier future removal. Concrete is recommended for most permanent installations.
Dig holes 3 times the post width. For a 4×4 post, dig a 12-inch diameter hole. For a 6×6 post, dig an 18-inch hole. This allows room for concrete and proper alignment.
Yes, in well-draining compact soil. Use the "dry pack" method: tamp gravel and soil in 4-inch layers. This works for temporary fences or when you may need to move posts later. Not recommended for gates or heavy loads.
Concrete sets in 24-48 hours but takes 7-28 days to fully cure. Wait at least 24 hours before attaching fence panels, and 2-3 days for heavy gates. Faster in warm weather, slower in cold.
Yes, always slope concrete away from the post at the top. This sheds water and prevents rot by keeping moisture from pooling around the wood. Create a 5-10% slope with a trowel while the concrete is wet.
Cedar and redwood naturally resist rot (20-30 years untreated). Pressure-treated pine is most common (15-20 years). Black locust and white oak are extremely durable (30+ years) but expensive. Always use ground-contact rated lumber for buried portions.
🚨 CRITICAL — RULE OF THUMB ONLY, NOT ENGINEERING:
Multipliers (1/4, 1/3, 1/2 of fence height) are rough rule-of-thumb estimates, NOT engineering. Actual depth depends on factors this tool ignores:
BEFORE building: (1) Check your local frost depth (county extension office), (2) Account for wind exposure, (3) Verify hole diameter for concrete.